I've hurt both of my shoulders over the years, and nearly every doctor says the same thing...
Take Motrin to reduce the inflammation until you find out if it's muscular, tendon or joint.
Only
ICE an injury immediately after injuring it, after that, heat.
Take a few days off, and when you decide to hit the gym do some light stretching and go easy on the weight.
I would suggest omitting overhead presses for a few weeks minimum to allow the area to fully heal.
Do lateral raises to the side and rear with light weights as long as you don't feel any pain.
If you're lucky it's just "irritated" and not damaged.
Whatever you do, don't ignore it. If you keep doing the movements that hurt, you'll be sidelined for weeks, months or for good.